I just cross-posted a question and feel slightly dirty. It seems to me that almost any question here could also be posted there. The community there is so much bigger, unless the question is REALLY specific, I'd pretty much always be inclined to post there first.
Here is our discussion of when a programming question belongs here or SO. (And presumably also things like where an art question belongs here or the eventual art SEs, sysadmin here or Server Fault, etc.)
Here is our discussion of cross-posting. The short version: Don't.